1. Field
The present application relates to optical metrology, and, more particularly, to drift compensation for an optical metrology tool.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine a feature of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a structure on a semi-conductor wafer an optical metrology tool is used to determine the profile of the structure. By determining the profile of the structure, the quality of the fabrication process utilized to form the structure can be evaluated.
As a result of the broad adoption of optical metrology, one fabrication facility or site where microelectronics are manufactured typically has multiple optical metrology tools in a fleet whose results are used somewhat interchangeably. In these cases, it is desirable that the instruments in the fleet match one another. In the ideal case, if the instruments were identical, their measurements would match to some uncertainty determined by measurement noise. However, optical metrology tools show deterministic differences, where the difference between the measurements is greater than the uncertainties of the measurement. One approach to improve matching is to carefully calibrate the tools, so that the optical characteristics measured by tools are as similar as possible, even if the details of each of the tools construction dictate that the detected intensities on the same sample are different. In some sense, this is the goal of calibration.
Calibration is typically done with calibration structures, with the intention that the calibration will remain valid for measurements on various application structures. Often calibration structures are one or more thicknesses of an oxide on a silicon substrate. Application structures can be very different than these simple calibration structures. In a fabrication facility, one application structure can be resist on top of a stack of layers for the formation of transistor gates after it has been exposed and developed, in order to examine the effects of adjusting, e.g., focus and dose on the exposure tool. Another application structure can be shallow isolation trenches in a silicon substrate. In general, the optical characteristics of these application structures can be substantially different from one another, and from the optical characteristics employed in calibration, which is ideally intended to be valid for all structures.
However, even after the optical metrology tools in a fleet have been calibrated, their optical characteristics, and subsequently their fit parameters, can differ. Such differences can be an issue for the control of processes in the fabrication facility. Accordingly, it is desirable to compensate for variations in the optical characteristics of optical metrology tools for a given application.